Publish down estimate and reset bias when enabled the first time.

Change-Id: Ibe9f276c9882083ae30dbc3b5d4bc938619a3f35
diff --git a/frc971/control_loops/drivetrain/drivetrain.h b/frc971/control_loops/drivetrain/drivetrain.h
index 69e9794..1330708 100644
--- a/frc971/control_loops/drivetrain/drivetrain.h
+++ b/frc971/control_loops/drivetrain/drivetrain.h
@@ -62,6 +62,8 @@
 
   bool left_high_requested_;
   bool right_high_requested_;
+
+  bool has_been_enabled_ = false;
 };
 
 }  // namespace drivetrain